The present invention relates to a spark plug having electrically insulating oil between an insulator and a fitting for improving anti-fouling performance of the spark plug.
As is well known in the art, the spark plug includes a central electrode sheathed within a cylindrical insulator that is in turn held inside a cylindrical fitting while a distal end of the central electrode disposed adjacent to a distal end of the insulator faces a ground electrode secured to the fitting across a discharging gap. Electrical discharge and ignition take place at the discharging gap, i.e., on the distal end side of the insulator.
In such a spark plug, if a temperature of the igniting section is relatively low (for example, 50 degrees Celsius or below), conductive materials, such as carbons or the like, may adhere to the distal end side of the insulator, so that the insulation resistance is reduced, leading to reduced ignition performance of the spark plug. Particularly, this phenomenon tends to occur before a brand-new automobile is passed to an automobile user from an automobile manufacture or while the mileage of the automobile is below 1000 kilometers.
One proposal to inhibit such fouling is disclosed in Unexamined Japanese Utility Model Publication No. 60-160490. Within a gap defined between the insulator and the fitting in an interior of a distal part (on the igniting section side) of the fitting, first and second seal packings are arranged such that the second seal packing is positioned closer to the igniting section in comparison to the first seal packing. Furthermore, electrically insulating oil is filled in a section between the first and second seal packings within the gap.
A channel is provided through the second seal packing, which is positioned near the igniting section, to allow effluent of a small amount of the insulating oil through the channel toward the igniting section of the plug. With this construction, the small amount of insulating oil flowing from the channel to the igniting section forms a coating on the surface of the insulator in the high temperature environment, for example, during high engine speed operations, so that the reduction of the insulation resistance is advantageously inhibited, and disappearing of the insulating oil within a short period of time upon exposure to. the high temperatures is also inhibited, resulting in a long lasting anti-fouling effect of the insulating oil.
However, in the spark plug disclosed in the described Unexamined Japanese Utility Model Publication, two seal packings are required. Since the second seal packing disposed near the distal end of the fitting (near the igniting section) has a relatively complicated structure including the channel, the entire plug structure is disadvantageously complicated. On the other hand, if the second seal packing disposed near the distal end of the fitting (near the igniting section) is not provided, the insulating oil will disappear within a short period of time upon exposure to the high temperatures, and therefore the anti-fouling effect of the insulating oil will not last for a relatively long period of time.
The present invention addresses the described problems, and it is an objective of the present invention to provide a spark plug with a relatively simple structure that can retain electrically insulating oil within a gap defined between an insulator and a fitting for inhibiting the fouling of the insulator and that can maintain the anti-fouling effect of the insulating oil for a relatively long period of time.
The present invention is based on the fact that silicone oil, fluorine oil, or the like used as the insulating oil is liquefied and has a surface tension under the operating conditions (for example, at 200-300 degrees Celsius), so that if the gap defined between the insulator and the fitting becomes narrow enough, the insulating oil can be retained within the gap by the surface tension of the insulating oil.
To achieve the objective of the invention, a spark plug comprises a central electrode, an insulator disposed outside of the central electrode, a fitting disposed outside of the insulator and a ground electrode electrically connected with the fitting and also facing the central electrode. An annular seal member is disposed in a gap defined between the insulator and the fitting. Electrically insulating oil is filled in an oil-retaining section of the gap that is located on the distal side of the seal member. The oil-retaining section of the gap extends axially and continuously for a length of 2 mm or more. The oil-retaining section has a profile that allows retention of the electrically insulating oil within the oil-retaining section of the gap by a surface tension of the electrically insulating oil while the electrically insulating oil is in a liquid state.